1. Field of the Invention
The present invention relates to an overcurrent protection circuit used for a switching power supply.
2. Description of the Related Art
Switching power supplies used as power supplies for operating circuits of electronic units are classified into isolating-type and non-isolating-type switching power supplies. In the case of an isolating-type switching power supply, the switching power supply comprises an input circuit including a first coil at the primary side of a transformer as an input side and an output circuit including a second coil magnetically coupled with the first coil through a core, at the secondary side as an output side.
The input circuit includes the first coil, a DC power supply, a switch device, and a control circuit.
A current supplied from the DC power supply to the first coil flows intermittently by means of ON/OFF control of the switch device performed by the control circuit. As a result, an AC voltage is generated in the second coil, which is magnetically coupled with the first coil. This AC voltage is extracted as a DC output voltage by a rectifying circuit and a smoothing circuit provided for the output circuit, and supplied to a load.
A pulse-width-modulation IC (hereinafter called a PWM IC) is usually used for the control circuit. The PWM IC is basically formed of an error amplifier, a triangular-wave generator, and a comparator.
An output voltage V.sub.out from the output circuit is fed back to the inverted input terminal of the error amplifier and a constant reference voltage is applied to the non-inverted input terminal. The triangular-wave generator outputs, for example, a fundamental triangular wave having a frequency of 20 kHz to 2 MHz. The fundamental triangular wave is input to the inverted input terminal of the comparator and an amplified voltage V.sub.amp amplified by the error amplifier is input to the non-inverted input terminal. The comparator compares the fundamental triangular wave with the amplified voltage Vamp and generates a driving pulse for ON-control of the switch device during the period when the fundamental triangular wave is larger than the amplified voltage Vamp. Therefore, as shown in FIGS. 5A to 5C, when the amplified voltage V.sub.amp1 is large, the pulse width of the driving pulse becomes small, and when the amplified voltage V.sub.amp2 is small, the pulse width of the driving pulse becomes large. As a result, the ON period of the switch device is varied and pulse-width-modulation control is constantly applied to the output voltage V.sub.out from the output circuit.
The frequency of the driving pulse is the same as that of the fundamental triangular wave. In the PWM IC, the ratio of the pulse width of the driving pulse to the period of the driving pulse, namely the duty cycle, is determined. The duty cycle can be made small by reducing a voltage applied to a duty-cycle setting terminal provided for the PWM IC.
In general-purpose PWM ICs, an overcurrent protection circuit is not provided in many cases. Instead, an overcurrent protection circuit is separately provided for a switching power supply. By referring to FIG. 6, a switching power supply provided with an overcurrent protection circuit will be specifically described below with a resonance-reset-forward-type switching power supply being taken as an example.
An input circuit of the switching power supply includes a first coil 1A at the primary side of the transformer 1, described above, a DC power supply 2, a switch device 3, and a PWM IC 4. An overcurrent protection circuit 5 is also connected as shown.
In a switching power supply, MOS FETs (metal oxide semiconductor field effect transistors), IGBTs (insulated gate bipolar transistors), and bipolar transistors are used for the switch device 3, in general. Therefore, in the following description, a case in which a MOS FET is used as the switch device 3 will be taken as an example and described.
One end of the first coil 1A is connected to the positive voltage side of the DC power supply 2, and the other end is connected to the drain D of the MOS FET 3. The source S of the MOS FET 3 is connected to the negative voltage side of the DC power supply 2 through the overcurrent protection circuit 5. The gate G of the MOS FET 3 is connected to a gate drive terminal T.sub.1 of the PWM IC 4. A ground terminal T.sub.2 of the PWM IC is connected to the negative voltage side of the DC power supply 2.
The overcurrent protection circuit 5 is formed of a current detecting resistor 6, a comparator 7, a comparison-reference power supply 8, and a resistor 9.
One end of the current detecting resistor 6 is connected to the source S of the MOS FET 3 and to the inverted input terminal of the comparator 7. The other end of the current detecting resistor 6 is connected to the negative voltage side of the DC power supply 2. The non-inverted input terminal of the comparator 7 is connected to the positive voltage side of the comparison-reference power supply 8. The negative voltage side of the comparison reference power supply 8 is connected to the negative voltage side of the DC power supply 2. The open collector output of the comparator 7 is connected to the maximum-duty-cycle setting terminal T3 for the PWM IC 4 provided for the PWM IC 4 and also connected to a constant-voltage reference power supply V.sub.s through the resistor 9.
Usually, a capacitor 10 is connected in parallel to both ends of the current detecting resistor 6, and the capacitor 10 bypasses a high-frequency component included in a current detected by the current detecting resistor 6. As a result, malfunction of the overcurrent protection circuit 5 caused by the high-frequency component is suppressed. When the PWM IC 4 is provided with a constant-voltage output terminal T.sub.s, the open collector of the comparator 7 may be directly connected to the output terminal T.sub.s through the resistor 9. The comparison-reference power supply 8 may be formed by dividing the constant voltage output from the output terminal T.sub.s with resistors.
The output circuit includes a second coil 1B, an output rectifying diode 11, a flywheel diode 12, a choke coil 13, and a smoothing capacitor 14.
The second coil 1B is magnetically coupled with the first coil 1A through the core 1C. One end of the second coil 1B is connected to the cathode of the output rectifying diode 11. The anode of the output rectifying diode 11 is connected to the anode of the flywheel diode 12, and the cathode of the flywheel diode 12 is connected to the other end of the second coil 1B. The anode of the output rectifying diode 11 is also connected to one end of the choke coil 13, and the other end of the choke coil 13 is connected to the cathode of the flywheel diode 12 through the capacitor 14. The cathode of the flywheel diode 12 is connected to the feedback terminal T.sub.4 of the PWM IC 4.
A load 15 is connected to both ends of the smoothing capacitor 14.
The operation of the switching power supply will be described below.
Between both ends of the current detecting resistor 6, a voltage proportional to a source current I.sub.s flowing through the source S of the FET 3 is generated. The comparator 7 compares the voltage V.sub.6 across the current detecting resistor 6 with the reference voltage V.sub.8 of the comparison-reference power supply 8.
When the source current I.sub.s does not exceed the current specified in advance, the voltage V.sub.6 is smaller than the reference voltage V.sub.8, and the comparator 7 is kept at an off state. Therefore, a current does not flow from the reference power supply V.sub.s to the comparator 7 through the resistor 9 and the open collector of the comparator 7. Consequently, the voltage at the duty-cycle setting terminal T.sub.3 of the PWM IC 4 is maintained at the same voltage as that of the reference power supply V.sub.s, and the duty cycle of the PWM IC 4 is held at a constant.
When the source current I.sub.s becomes greater than the current specified in advance, in other words, when an overcurrent flows through the source S of the FET 3, the voltage V.sub.6 becomes larger than the reference voltage V.sub.8 and the comparator 7 is turned on. As a result, a current flows from the reference power supply Vs to the comparator 7 through the resistor 9 and the open collector of the comparator 7. Therefore, the voltage at the duty-cycle setting terminal T.sub.3 of the PWM IC 4 becomes lower than that of the reference power supply Vs by a voltage drop at the resistor 9. Consequently, the duty cycle of the PWM IC 4 is set to a small value, and the pulse width of the driving pulse for ON-control of the MOSFET becomes small. As a result, the overcurrent flowing through the source S of the MOSFET 3 is suppressed.
On-control is applied to the FET 3 by the driving pulse output from the gate drive terminal T1 of the PWM IC 4, and a current flows intermittently through the first coil 1A. As a result, an AC voltage is generated in the second coil 1B. This AC voltage is half-wave rectified by the output rectifying diode 11 and the flywheel diode 12, then smoothed by the choke coil 13 and the smoothing capacitor 14, and supplied to the load 15 as a DC output voltage V.sub.out.
In recent years, to make a switching power supply compact, however, a switch device operating at a relatively high switching frequency ranging from several hundreds to several MHz is used in some cases.
Therefore, in a case in which an overcurrent protection circuit using a comparator is employed, when the comparator response time is long, even if an overcurrent flows through the switching power supply, the comparator cannot immediately follow. As shown in FIG. 7A, the curve indicating the relationship between the output current and the output voltage decreases gradually. Therefore, the maximum value of the output current flowing through the switching power supply becomes large, and heat generated at electronic components constituting the input circuit and the output circuit becomes high.
On the other hand, when a comparator having a short response time is used so as to follow the operation of the switch device, the curve indicating the relationship between the output current and the output voltage, obtained when an overcurrent flows through the switching power supply, decreases almost immediately. Therefore, a response to an overcurrent becomes extremely satisfactory. However, a short-response-time comparator is generally expensive and it has a large power consumption. Therefore, the switching power supply becomes expensive and energy is wasted.